PROJECT SUMMARY/ABSTRACT Tendon and ligament injuries account for nearly half of all musculoskeletal injuries each year in the United States. Even with considerable improvements, surgical repairs do not consistently restore physical function of injured tissues. As a result, there are lengthy delays or failures to return to pre-injury activities, thus posing a substantial clinical, economic and social burden. Poor surgical outcomes primarily result from excessive inflammation and insufficient regeneration of repaired tissues. Therefore, the long-term goal of this study is to develop biologic therapies to enhance tendon repair. With recent advances in regenerative medicine and extracellular vesicle (EV) research, the proposed project has been designed to investigate the mechanistic role and clinically relevant application of stem cell EVs in tendon repair. EVs are membrane-enclosed nanoparticles that mediate intercellular communication by transferring bioactive molecules (proteins, RNAs, etc.) from cell to cell. Although EVs from adipose-derived stem cells (ASCs) have been found to modulate tissue inflammatory response and to promote regeneration of some soft tissues, their role in tendon repairs has not been directly explored. Aim 1 will use co-culture models to determine the role and underlying molecular mechanism of ASC EVs in regulating macrophage inflammatory response and tenocyte activity and function in the context of tendon injury and repair. Aim 2 will use a clinically relevant mouse Achilles tendon injury and repair model to determine the therapeutic efficacy of ASC EVs in reducing inflammation, increasing tendon matrix regeneration and restoring structure and strength of injured tendons. Results from Aim 1 will reveal new insights into the mechanism of ASC EVs in regulating tendon healing process and provide guidance for further engineering of defined components within ASC EVs to provide improved therapeutic efficacy and safety. Positive outcomes from Aim 2 will prove the concept of ASC EV-based therapy for tendon injury as well as other related disorders and lead to clinical studies, thus contributing to the improvement of musculoskeletal health.